Recurrent arc heating system

ABSTRACT

Gas or fluid flows through a gap between electrodes having an arc therebetween at a very high velocity while a system voltage is continuously maintained sufficient to cause breakdown at the gap. The high velocity gas elongates the arc until the arc voltage required for electrical conduction exceeds the breakdown voltage of the gap whereupon sparkover occurs in the gap, the arc being thereafter elongated again by the gas passing through the gap until the voltage required to sustain arcing exceeds the breakdown voltage of the gap, the cycle of gap breakdown and elongation being repeated over and over again. The greatly extended arc provides for more efficient heating of the gas, better mixing and a more uniform temperature to which the gas is heated.

1 "r -17 73 on 3174611830 llmte States Patent 1191 1111 3,746,830 Fey et al. July 17, 1973 RECURRENT ARC HEATING SYSTEM 3,400,070 9/1968 Naff 313/231 x E l3 6 [75] Inventors: Maurice G. Fey, Turtle Creek; ir 1 9 Charles "Win; Frederick 3,279,177 10/1966 Ducati 219/121 P x Azinger, J12, Pittsburgh; George A. Kemeny Export of Primary Examiner-lt V. Truhe [73] Assignee: Westinghouse Electric Corporation, Assistant Examiner-Gale R. Peterson Pittsburgh, Pa. Attorney-A. T. Stratton, M. l. Hull et al. [22] Filed: Mar. l5, 1971 211 Appl. 110.; 124,517 [57] ABSTRACT Related Us Application Data Gas or fluid flows through a gap between electrodes having an arc therebetween at a very high velocity [62] Division of while a system voltage is continuously maintained sufficient to cause breakdown at the gap The high velocity gas elongates the arc until the arc voltage required for [5:12] F 9/121 313/23 electrical conduction exceeds the breakdown voltage {2 'gzi P of the gap whereupon sparkover occurs in the gap, the

. are being thereafter elongated again by the gas passing 313/231 315/111 through the gap until the voltage required to sustain References Cited arcing exceeds the breakdown voltage of the gap, the

cycle of gap breakdown and elongation being repeated UNITED STATES PATENTS over and over again. The greatly extended arc provides 3,474,279 10/1969 Kemeny et al. 219/121 R X for more efficient heating of the gas, better mixing and 2,919,370 12/1959 Giannini et al. 1 313/231 3 more uniform temperature to which the gas is heated 3,52l,l06 7/1970 Hess 219/121 R X 3,361,353 l/1968 Gillman 313/231 X 2 Claims, 9 Drawing Figures Patented July 17, 1973 4 S he ets-Sheet 1 4 sheets-sheet 2 FIG. 2

6 RM m N R E u A CURRENT SOURCE 4 Sheets-Sheet 4 Fl G5.

lll I04 "0 p2 P AP CONTROL AND P R SUP ' FIF COlL PROCESS CONTROL WE U JUSTA 1 RECURRENT ARC HEATING SYSTEM CROSS REFERENCE TO RELATED APPLICATIONS This application is a division of copending application Ser. No. 790,417, filed Jan. 10, I969, now US. Pat. No. 3,629,553, by Maurice G. Fey et al, entitled Recurrent Arc Heating Process, and owned by the present assignee.

This application is related to the application of K. H. Yoon et al. for An Arc Heater With A Spirally Rotating Arc, Ser. No. 764,090, filed Oct. 1, 1968, and assigned to the assignee of the instant invention.

DESCRIPTION OF THE PRIOR ART BACKGROUND OF THE INVENTION Prior art processes for heating gas employing a pair of spaced annular electrodes with an arc therebetween have a number of problems associated with proper operation. For example, gas has usually been admitted through the gap between electrodes at such a low velocity that arcing frequently occurs from an electrode to the structure enclosing the space between electrodes. Prior art processes have been limited in the gas velocity which could be used because high velocity resulted in instability and arc extinction. Additionally, the arc has usually been confined to an annular path around and between annular spaced electrodes rather than being rapidly blow to an elongated position in the gas to be heated, with the result that heat efficiencies have usually been low. Additionally, prior art processes have usually been conducted at such low power factors that they were completely unacceptable in commercial practice.

SUMMARY OF THE INVENTION These and other disadvantages of prior art processes for heating a gas or fluid have been overcome in the processes of our invention. With the short gap employed in our invention the no-load or system voltage can be reduced from that required in prior art processes while at the same time the average arc voltage is increased, resulting in a substantially improved power factor. The increased average arc voltage of our process results from elongation of the are by forcing gas through the gap at very high velocity. Furthermore, in our process, the arc is so elongated that when rotated by a magnetic field the arcing zone may include a very large area and the arc may extend substantial distances from the gap itself, resulting in greater heating efficiency, better mixing and more uniform temperature of the heated gas. Our process may be practiced with direct current; where alternating current is employed, the periodic current zeros insure that the arc will be returned to the gap at least once each alternation. The high velocity gas keeps the gap free of ionized material thereby maintaining integrity of electrical insulation and maintaining the gap breakdown voltage at a high value.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a cross-sectional view through means forming an enclosed arcing zone suitable for practicing the process of our invention;

FIG. 2 is a cross-sectional view through additional means forming an enclosed arcing zone suited for practicing the process of our invention wherein the width of the electrical breakdown path may be adjusted;

FIG. 3 is an electrical circuit diagram of the circuit supplying an are by which our inventive process is practiced by alternating current;

FIG. 3A is a fragmentary circuit diagram where our process is practiced with direct current;

FIGS. 4A and 4B are oscillograms showing sample instantaneous arc voltage in a test run employing the process of our invention, the time base of both FIGS. 4A and 4B being 200 microseconds per division;

FIG. 5 is a view of means for automatically adjusting the width of the electrical breakdown path in accordance with a control system of our invention which utilizes the difference between the pressure in the area enclosing the arc zone and the pressure of the gas before it enters the arc zone to adjust the width of said breakdown path;

FIG. 6 is a view of a system for automatically adjusting the current in field coils which supply magnetic fields to rotate the arc within the arcing zone, it having been found that within limits the breakdown or sparkover voltage in the gap and resultant average arc voltage are functions of magnetic field strength, and that enthalpy sensitive means measuring the temperature rise in the heated gas may be employed to automatically adjust the magnetic field current to maintain the desired arc voltage and thereby maintain optimum enthalpy of the heated gas; and

FIG. 7 is a graph further assisting in illustrating the processes described in connection with FIGS. 1, 2, 3, 4A and 4B.

DESCRIPTION OF THE PROCESSES It will be understood that no particular arc heater configuration is essential to practicing the processes of our invention. The term arc heater" is employed throughout the specification and claims merely as a matter of convenience and not in a limiting sense. Essential to practicing the processes of our invention-are an electrical breakdown path; the gas or fluid to be heated is forced through the path at a very high velocity elongating'the are formed therein; the system voltage is maintained at a value which will cause breakdown'in the breakdown path when the arc has been extended to such a length that the arc voltage exceeds the breakdown voltage; the gas to be heated is channeled through the zone of the elongated arc.

With particular reference to FIG. 1, reference numeral 11 generally designates an enclosed area herein called an arc chamber defined in part by two annular axially spaced electrodes 12 and 13 having an axial gap 14 therebetween. Arc 15 is seen extending between electrodes and it is seen that each of the electrodes is fluid cooled as by passageways l6 and 17 and that each of the electrodes has a magnetic field coil therein, these being designated 18 and 19 and setting up a magnetic field transverse to the arc path while in the gap which causes the arc 15 to rotate substantially continuously around the electrodes. The aforementioned passageway in electrode 12 for cooling fluid 16 is seen to communicate at the ends thereof with fluid headers 20 and 21 and the aforementioned cooling passageways 17 in electrode 13 is seen to communicate at the ends thereof with fluid headers 22 and 23.

In addition to the aforementioned electrodes 12 and 13, the arc chamber 11 is seen to be bounded or defined in part by a heat shield generally designated 25 with a passageway 26 between the outside wall of the heat shield and the inside wall of the electrode communicating with a fluid header 27 so that gas may be injected in an annular path between the heat shield and the electrode. This supplemental means for injecting gas into the arc chamber may or may not be used where the process of our invention is employed, and may be dispensed with if desired.

The upstream end of the chamber is seen to be closed by a plug 29, and additionally gas or fluid may be injected between the plug and the adjacent wall of the heat shield 25 in an annular path from gas header 31. Furthermore, it will be understood that in practicing the processes of our invention gas may or may not be injected through this last mentioned passageway, which may be omitted if desired.

Extending adjacent the downstream electrode 13 is a downstream heat shield generally designated 33 with means which may or may not be employed in the process of our invention for injecting gas from header 34 in an annular path between heat shield 33 and electrode 13. A nozzle generally designated 36 completes the structure with means which may or may not be employed if desired for injecting gas in an annular path between nozzle 36 and heat shield 33 from gas header 37. Fluid injection or other gases or solids at these locations may be used for the purpose of quenching nonequilibrium chemical reactions.

Enclosing the space between electrodes 12 and 13 is an arc chamber wall generally designated 39 which may be composed of insulating material, and it is seen that there are gas headers 41 and 42 at the upstream and downstream ends of the chamber wall 39 for admitting gas into the space 43 from whence it flows through the gap 14 into the arc chamber 11.

It will further be understood that energizing the magnetic field coils l8 and 19 causes the are to rotate in an annular path around the electrodes, and the arc is elongated by aerodynamic forces of the high velocity gas entering through space 14 to a position indicated by the path 15a and may travel down the sides of the electrodes 12 and 13. Merely for purposes of description it will be assumed that the are path 15a represents an elongation of the arc to a position whereat the arc voltage equals the breakdown voltage of gap 14 whereupon the gap breaks down and the are is again initiated in the gap at path 15 and is immediately blown by the high velocity gas toward the center of the arc chamber.

Particular reference is made now to FIGS. 4A and 4B which show oscillograms of the arc voltage in methane gas. It is seen from FIG. 4A that the are, after being ignited in the short gap, is extended to a length where the arc voltage becomes approximately 2,000 volts whereupon a sparkover occurs at the minimum gap, the sparkover being designated by the point m". It will be understood that the arc is extremely dynamic and that when it is moved by the high velocity gas it may on occasion follow different paths, one of these paths being indicated by the path 15b of FIG. 1. While the breakdown voltage of the gap may be normally 2,000 volts, it will be understood that the actual breakdown at the gap is statistical and that the breakdown voltage may be at l,200 volts, l,500 volts or 1,800 volts, and that the breakdown is very irregular. A breakdown at less than 2,000 volts is indicated by the point n" on the curve of FIG. 40. There are many causes for reduction in the breakdown voltage; when the arc is extended in path 15b the arc emits photons into the gap which give it an effective conductivity thereby reducing breakdown voltage, whereas if the arc is far away and spread down the electrodes as in arc path 15a the view factor is not as high and less photons will be emitted into the gap and the breakdown voltage of the gap may be higher when the arc takes path 15a than it is when the arc takes path 1512. Another factor contributing to the effective breakdown voltage is the pressure, as illustrated by the curve of FIG. 7, which is Paschens breakdown curve for a given gas and electrode surface conductivity. Both of the scales are logarithmic, in which E, the breakdown voltage, is plotted as a function of PD, where P is the pressure, and D is the distance between the electrodes.

It will be understood that the breakdown factor may be altered by changing the electrode shape.

Another factor affecting the breakdown voltage of the gap is that the high velocity gas entering through the narrow gap between electrodes may induce hot gases to recirculate back into the gap area and sparkover may not occur at the minimum gap but between some points near the minimum gap, and hence the breakdown occurs at a lower voltage than the normal breakdown voltage.

It will be readily understood that the arc is extremely dynamic in nature and has normal fluctuations of a somewhat random nature.

Furthermore, the arc may become so extended by the pressure of gas passing through the gap 14 at very high velocities that the arc is twisted in a spiral path and may short out a portion of itself so that the actual are voltage returns to a value considerably higher than the sparkover voltage, such a condition being indicated by the point 0 of the graph of FIG. 4A. A breakdown of the gap at a voltage less than the normal voltage appears to take place at point p; a breakdown of the gap after the arc has been elongated to where the arc voltage is 2,000 volts apperas to take place at point q of FIG. 4B; point r may represent a shorting out of a portion of the are by the arc itself as may point s, while point t may represent a breakdown at the gap due to the increased conductivity of photons and point u may result from the are shorting out itself as it follows a twisted path as it is elongated by gas blasting through the arc gap.

Particular reference is made now to the electrical circuit of FIG. 3. Electrodes generally designated 12' and 13' have a gap 14 therebetween with are following paths, which are exemplary, at 15' and 15", the field coils being designated 18' and 19 respectively. Electrode 13 is connected by way of lead 55 to one terminal of a ballast reactor 50 which is connected by way of lead 47 and circuit breaker contacts 49 to one terminal 52 of an alternating current generator of for example 2,300 volts A.C. having the other terminal 51 thereof connected by way of circuit breaker contacts 48 and lead 46 to the aforementioned electrode 12. The aforementioned system voltage is that between leads 46 and 47. The are heater of FIG. 3 is also seen to include a closure plug generally designated 29.

FIG. 3A is a fragmentary circuit diagram where our process is practiced with direct current. Ballast reactor 50 is replaced by resistor 56, and terminals 51' and 52' are connected to a direct current source.

As previously stated, within certain limits the arc voltage can be increased by increasing the field coil current. To this end, a sensing device 128 is provided which may sense changes in the enthalpy of the gas heated in arc chamber 125. If desired, sensing device 128 may be a water cooled calorimetric probe in which the temperature rise of fluid passing through the probe is measured, this temperature rise being a function of the enthalpy of the gas. Probe 128 supplies a signal by lead 129 to process control apparatus shown in block form at 130 which supplies control signals by lead or leads 131 to field coil power supply 132 shown in block form. The power to field coils 135 and 136 in electrodes 117 and 118 respectively is conducted by lead means 137 extending from the field coil power supply 132 to the respective field coils. Whereas only one lead 137 is shown for simplicity of illustration and to indicate the control function of the field coil power supply it will be understood that two leads may extend from each field coil to the field coil power supply 132 to provide acomplete electrical circuit.

Both of the systems of FIG. and FIG. 6 permit in effect a self-regulating process in which the parameters of gap length and magnetic field strength are automatically adjusted to maintain optimum conditions for maximum heating of the gas blasted into the arc chamber through the short gap, and also for the most uniform heating of all portions of the gas to the same temperature.

Other types off electrode configurations, other than the axially spaced annular electrodes selected for purposes of illustration, may be employed in practicing the method of our invention. For example, the process of our invention may be practiced with a pair of coaxially aligned radially spaced electrodes providing an annular breakdown path in which the are before elongation extends radially between electrodes, such an electrode configuration being described and claimed in the copending application of A. M. Bruning et al. for Cross Flow Arc Heater Apparatus and Process for the Synthesis of Carbon, Acetylene, and Other Gases", Ser. No. 507,345, filed Nov. 12, 1965, now U.S. Pat. No. 3,554,715, and assigned to the assignee of the instant invention. Additionally, the process of our invention may be practiced employing a three phase alternating current source in which axially spaced heat shield rings serve as electrodes, such a configuration being described and claimed in U.S. Pat. No. 3,309,550 issued Mar. 14, 1967 to C. B. Wolf et al. for Multiple Annular Electrode Gas Arc Heater With A Magnetic Arc Spinner", and assigned to the assignee of the instant invention.

By way of further summary, our process includes the steps of forcing a gas to be heated through a relatively short gap between a pair of electrodes while maintaining a system voltage at all times sufficient to cause breakdown of the gap between electrodes, the very high velocity gas causing periodic elongation of the arc to a length whereat the arc voltage exceeds the gap breakdown voltage whereupon the gap breaks down and the process of arc elongation is periodically repeated, this periodic breakdown and elongation usually occurring many times per alternation of the alternating current supplying the are as illustrated in FIGS. 4A and 4B are at least once following current zero of the alteranting current, and our process because of increased turbulence resulting from rapid power fluctuations provides for more efficient heating of the gas, better mixing and a more uniform temperature to which the gas is heated.

Our process also provides improved gas mixing because of pressure pulsations caused by perturbations in power input. Instantaneous power is current times arc voltage; the recurring gap breakdown and are elongation in effect converts a 60 cycle sinusoid into a saw tooth wave having between five and twenty times the frequency of the alternating current.

Since the arc roots are periodically moved from the short gap to theinside walls of the electrodes, the surface area of the electrodes contacted by the arc roots is greatly increased with a corresponding reduction in erosion, providing an increase in total electrode life.

Since average arc voltage is greatly increased, for a given power input are current can be reduced, the reduction in arc current decreasing electrode erosion rate exponentially.

Increased gas throughput decreases gas residence time, therefore the heating efficiency of our process is greatly improved.

As long as the system voltage exceeds the gap breakdown voltage, operation is perfectly stable and is free from arc-outs.

No fuse is needed to initiate arcing. Sparkover initiates the are.

Our process eliminates spurious arcing to heat shields; in fact there need not be any heat shields. All of the arc heater interior surfaces (except the end'plug) may be electrode arcing surfaces.

In our process, the arc can not be extinguished by high gas flow; therefore there is no upper boundary on gas flow rate.

Our process may employ either alternating current or direct current to produce the arc. In direct current operation, pressure perturbations rather than periodic reversals in the direction of arc rotation produce gas stirring at mixing.

Whereas the oscillograms ofFIGS. 4A and 4B were obtained in heating methane, it will be understood that our process can beat any gas, or any gas mixture, such as air.

The foregoing written description and the drawings are illustrative and exemplary only and are not to be interpreted in a limiting sense.

We claim as our invention:

l. A system for automatically adjusting the gap length between a pair of spaced annular electrodes forming an arc chamber in an arc heater having an arc in the chamber into which gas is injected from a space surrounding and communicating with the gap at a high velocity through the gap thereby elongating the are which takes place between electrodes which system comprises, the chamber having a closed end and a gas outlet nozzle, means in the chamber adjacent the closed end for sensing the pressure of gas therein, means in the space for sensing the pressure of gas admitted into said space, both of said sensing means supplying output signals, means for applying both of said output signals to a differential pressure control circuit, a reversible motor controlled from said differential pressure control circuit, and means operatively connected to said motor for increasing the gap length between electrodes when the motor turns in one direction and decreasing the gap length between electrodes when the motor turns in the other direction.

The following is a detailed listing of test data for 3 operating conditions of our process employing a short gap heater to heat gas;

Case l Case 2 Case 3 Test No. H338 H368 Nl7l Process Gas CH, CH INl Arc Current (amps) 3250 2000 2000 Average Arc Voltage (volts) 715 870 205 Gas Throughout (lbm/sec) 0.51] 0.726 0.577 System Voltage (volts) 2080 3500 3520 Thermal Efficiency 72.6 84.0 62.3 Enthalph (Btu/lbm) 2830 I740 642 Velocity at Minimum Gap (ft/sec) 1072 I040 825 Minimum Gap Length (in.) 0.054 0.071 0.071

The aforementioned oscillograms of FIGS. 4A and 48 were taken showing instantaneous arc voltage during test No. H338 (Case 1). As previously noted from a description of the oscillograms, the arc voltage rises to about 2,000 volts whereupon sparkover occurs in the minimum gap. The process usually repeats itself several times during each half cycle of the alternating current. When the alternating current goes through current zero following each alternation, breakdown thereafter may occur in the gap. It may be noted that no-load or system voltage in Case 2 is higher than the 2,300 volts system voltage of FIG. 3 which permits stable operation during the practice of our process, and that system voltages well above the required voltage may be employed.

Further reference is made now to FIG. 2 which shows an additional arc heater which may be employed in the practice of our invention. Electrodes 12" and 13 are seen to have annular flange portions 62 and 63 having secured thereto peripherally spaced tubes or rods which are internally threaded, two of these rods or tubes being shown at 64 and 65, and two of these tubes or rods being shown at 66 and 67; extending into the aforementioned tubes or rods 64 and 67 inclusive are threaded members 68 and 71 inclusive which in turn extend into opposite ends of two tubes or rods 73 and 74 which have secured thereto for rotation therewith the pinions 76 and 77 respectively. Extending around the entire structure and meshing with the gears 76 and 77 is an annular gear wheel generally designated 79 which when turned in a counterclockwise direction causes the distance of the gap to change in one direction and when turned in a clockwise direction causes the gap distance to change in the other direction. There are seen to be peripherally spaced springs 81 and 82 seated in bores 83 and 84 in members 85 and 86 slidably disposed with respect to members 87 and 88, which may be one member extending around the entire arc chamber, the peripherally spaced springs including springs 81 and 82 tending to maintain the gap between electrodes 12 and 13' at the maximum length permitted by the position of the aforementioned gears 66 and 67 and the threaded members 68, 69, 70 and 71. The aforementioned gear wheel 70 may be driven by a motor, not shown for convenience of illustration, located adjacent the arc heater and having a gear thereon which meshes with the teeth 90 of member 79 or if desired a motor may be located at the plug end of the arc heater and secured thereto with extended gearing from the motor meshing with the aforementioned gear wheel 79.

In the arc heater of FIG. 2, the gas headers 91 and 92 gation of the are when it had reached an arc voltage substantially equal to the breakdown voltage of the gap 14" is shown by the arc path 150.

Particular reference is made now to FIG. 5, a system for automatically regulating the width of the gap in accordance with the difference in pressure between that inside the arc heater and that in the passageway on the outside of the gap through which as aforementioned the gas is blasted at very high speed. The electrodes are shown diagrammatically at 95 and 96, the arc path being exemplified at 115; members 98 and 99 slidable with respect to each other represent means for permitting the gap length to be varied while maintaining gas tight the space 100. It will be understood that at least one end of the space 100 there is a gas header, not shown for convenience of illustration, from which gas passes through the gap between electrodes at very high velocity. Means for sensing the pressure in the area enclosing the arc zone is designated 101, and may be of any convenient type, and supplies a signal by lead or leads 102 to a differential pressure control, shown in block form at 103, supplying an output to a motor 104. An additional sensor 106 is located within the aforementioned space 100 and supplies a signal by way of lead or leads 107 to differential pressure control block 103 so that at all times a signal corresponding to the pressure in space 100 and another signal corresponding to the pressure in the enclosed area or are heater, are supplied to the control circuit 103. It will be understood that the block 103 contains a power source for motor 104, or has supplied therethrough power for motor 104. The aforementioned motor 104 is reversible and has a driven gear 109 meshing with gear 110 secured to a threaded member 111 which passes through threaded studs 112 and 113 attached respectively to the aforementioned annular members 98 and 99, which members it is understood are secured to the electrodes 95 and 96 so that rotation of the gear 109 in one direction, causes the gap to be shortened, whereas rotation of the gear 109 in the other direction causes the gap between electrodes to be lengthened. It will be understood that such regulation of the gap length is desirable to insure that the are illustrated at 115 is successively elongated to a value at which the arc voltage exceeds the breakdown voltage of the electrode gap. Electrodes 95 and 96 may have magnetic field coils therein, not shown for convenience of illustration.

Particular reference is made now to FIG. 6. The electrodes are shown symbolically at 117 and 118 having outwardly extending falnge portions 119 and 120 respectively with an annular insulating member 121 spacing the flange portions a predetermined distance from each other and thereby providing a gap length of predetermined distance between the annular electrodes. Spacer member 121 is composed of insulating material, and defines a space 122 through which it is understood gas is brought to the gap from one or more gas headers, not shown for convenience of illustration, and passed at high velocity through the short gap between electrodes from outside the electrodes toward the inside of the arc chamber. Leads 123 and 124 connect the electrodes across the source of potential. For simplicity of illustration, the electrode 120 is shown as also enclosing the upstream end of the arc chamber 125 in which the arc 126 takes place.

2. A system for use with an arc heater having a pair of spaced electrodes with a gap therebetween across which an are initially takes place, the arc heater having means for bringing gas into a space for passing said gas at high velocity through the gap between electrodes thereby removing the arc from the gap and elongating the arc between electrodes, each of said electrodes having a field coil mounted therein for creating a magnetic field which causes the elongated arc to move substantially continuously in path forming an elongated arc zone, the arc heater having a gas exhaust nozzle, comprising: means at the exhaust nozzle for sensing the temperature of gas heated in the arc heater to thereby represented by variations in said signal. 

1. A system for automatically adjusting the gap length between a pair of spaced annular electrodes forming an arc chamber in an arc heater having an arc in the chamber into which gas is injected from a space surrounding and communicating with the gap at a high velocity through the gap thereby elongating the arc which takes place between electrodes which system comprises, the chamber having a closed end and a gas outlet nozzle, means in the chamber adjacent the closed end for sensing the pressure of gas therein, means in the space for sensing the pressure of gas admitted into said space, both of said sensing means supplying output signals, means for applying both of said output signals to a differential pressure control circuit, a reversible motor controlled from said differential pressure control circuit, and means operatively connected to said motor for increasing the gap length between electrodes when the motor turns in one direction and decreasing the gap length between electrodes when the motor turns in the other direction.
 2. A system for use with an arc heater having a pair of spaced electrodes with a gap therebetween across which an arc initially takes place, the arc heater having means for bringing gas into a space for passing said gas at high velocity through the gap between electrodes thereby removing the arc from the gap and elongating the arc between electrodes, each of said electrodes having a field coil mounted therein for creating a magnetic field which causes the elongated arc to move substantially continuously in path forming an elongated arc zone, the arc heater having a gas exhaust nozzle, comprising: means at the exhaust nozzle for sensing the temperature of gas heated in the arc heater to thereby obtain a signal varying with variations in the enthalpy of the gas, means for applying said signal to process control apparatus adjusted in accordance with other operating parameters of the arc heater, the process control apparatus being operatively connected to adjustable field coil power supply means, circuit means operatively connecting the field coil power supply means to the field coils within the two electrodes to regulate the field coil current and thereby the strength to the magnetic field in accordance with changes in the enthalpy of the gas emerging from the arc heater as represented by variations in said signal. 